In the field of access control and monitoring, cards having magnetically-encoded data have been known and used for one or both of two functions, i.e., (1) as a key for unlocking a lock, and/or (2) for providing information (e.g., employee identification, time of arrival and/or departure) to a central control or monitoring station.
By way of example, U.S. Pat. Nos. 2,648,729 RE. 27,753 and 2,931,953 show locks having tumblers (i.e., locking elements) in the form of magnets which are adapted to be unlocked by a key in the form of a card having ferrous shims (e.g., see U.S. Pat. No. 2,648,729), magnetized spots (e.g., see U.S. Pat. No. 3,611,763) or magnets (e.g., see U.S. Pat. No. 2,931,953) therein.
The prior art literature also shows card readers for detecting or reading cards having information encoded thereon. The information on the cards is typically magnetically encoded, and the detectors or sensing elements in the card readers may be sensing transformers (e.g., see U.S. Pat. No. 3,422,252), magnetic coils (e.g., see U.S. Pat. No. 3,717,749), Hall effect devices (e.g., see U.S. Pat. No. 3,634,657), reed switches (e.g., see U.S. Pat. No. 3,581,080) and other switching devices.
It is also known in the prior art to combine locks having magnetic tumblers (e.g., as shown in U.S. Pat. No. RE 27,753) with card readers. For example, see U.S. Pat. No. 3,581,030 which combines a programmable lock such as shown in U.S. Pat. No. RE. 27,753 with a card reader utilizing reed switches as the sensing elements.
While the prior art card readers (such as those described above, for example) have met with some acceptance in the market place, there are problems and/or drawbacks associated with each.
For example, the reed switch type readers (such as disclosed in U.S. Pat. No. 3,581,030) are relatively expensive and somewhat complex to assemble. All of the reed switches, for example, must be precisely aligned and hand soldered in place. However, the switches must be calibrated, checked, recalibrated and rechecked to ensure reliable operation. Still further, such reed switches are of relatively small reliability when used with magnetically encoded cards whose magnetic spots are of relatively low magnetic strength (e.g., less than 100 gauss).
The prior art card readers which have employed Hall effect devices (such as shown, for example, in U.S. Pat. No. 3,634,657) have utilized Hall effect cells which are either (1) highly sensitive and relatively expensive, or (2) incapable of reading magnetized spots or elements having relatively weak magnetic fields (e.g., below about 100 gauss). Thus, in order to provide magnetically-coded cards, such as the card shown in U.S. Pat. No. 3,611,763, for example, having magnetized spots of sufficient magnetic strength to trigger or actuate a Hall effect device of the type shown in U.S. Pat. No. 3,634,657, it is necessary to purchase relatively expensive magnetizable material for the cards. By way of example, the cost of the magnetizable material for use in key cards having magnetized spots of sufficient magnetic strength to trigger a relatively insensitive Hall effect cell may be four times as great as the cost of the same type magnetizable material which cannot be magnetized with sufficient magnetic strength to be read by insensitive cells.